module Control.Monad.State.Persistent( PersistentStateT
, PersistentState
, store
, runPersistentStateT
, runPersistentState
) where
import Control.Monad.State
import Control.Monad.Identity(Identity(..))
import Data.List.NonEmpty(NonEmpty(..),(<|),toList)
newtype PersistentStateT s m a =
PersistentStateT { runPersistentStateT' :: StateT (NonEmpty s) m a }
deriving (Functor,Applicative,Monad)
store :: Monad m => PersistentStateT s m ()
store = PersistentStateT $ do
ss@(s :| _) <- get
put (s <| ss)
instance Monad m => MonadState s (PersistentStateT s m) where
state f = PersistentStateT $ do
(s :| os) <- get
let (x,s') = f s
put (s' :| os)
return x
runPersistentStateT :: Functor m => PersistentStateT s m a -> s -> m (a,s,[s])
runPersistentStateT (PersistentStateT act) initS = f <$> runStateT act (initS :| [])
where
f (x,ss@(s :| _)) = (x, s, reverse $ toList ss)
type PersistentState s = PersistentStateT s Identity
runPersistentState :: PersistentState s a -> s -> (a,s,[s])
runPersistentState act = runIdentity . runPersistentStateT act